Ontario Grain Farmer February 2021

20 MANAGING WEEDS IN Ontario farm fields is becoming more of a challenge due to herbicide resistance. Research is being conducted to find viable solutions. SEEDS BY THE MILLIONS A new waterhemp seed count helps explain why this invasive weed has taken hold inOntario — and why controlling it is such a challenge. In 2017, Peter Sikkema of the University of Guelph Ridgetown Campus embarked on a nine- year study to develop a diversified, integrated weed management strategy for multiple- herbicide-resistant waterhemp control in corn, soybeans, and winter wheat. From its initial, isolated discovery in Ontario around 2000 near Petrolia — suspected to have arrived on a waterhemp- contaminated combine from Illinois — this weed has gone on to become one of our farmers’ biggest problems. In 2014, glyphosate-resistant waterhemp was found on Walpole Island and has been confirmed in a dozen more counties. Research shows average yield loss from this weed is 17 per cent in corn and 43 per cent in soybeans. A first step for Sikkema was to determine how many waterhemp seeds were present in the test plots on two commercial farms, one near Cottam, and the other on Walpole Island. That way, he could later understand the effectiveness of the control strategies he will be developing, by counting the seeds present in fields that get treated. He didn’t know what to expect. But the numbers he found were staggering: 165 million seeds per acre in the Cottam plots and 16 million seeds per acre in the Walpole Island plots. “Those are huge numbers,” says Sikkema. “We had never done this kind of a study and I didn’t expect to find this many seeds. This is what some farmers face before they even start planting.” Sikkema’s nine-year study has a goal to deplete that seed by 95 per cent, using currently available technology and equipment. His study includes a variety of approaches. These include continuous soybean, two-crop rotations (corn-soybean and soybean-wheat) and three-crop rotations (corn-soybean-wheat and corn-soybean-wheat and cover crop), narrow 15-inch rows in soybean, and the use of efficacious herbicides. In the most diverse crop rotations, Sikkema will use 10 different herbicide active ingredients — dicamba, atrazine, bromoxynil, glyphosate, flumioxazin, pyroxasulfone, s-metolachlor, mesotrione, bicyclopyrone, and pyrasulfotole — from seven herbicide groups (4, 5, 6, 9, 14, 15, and 27) over a three-year period. Sikkema says that at both the Cottam and Walpole Island test sites, near-perfect multiple- herbicide-resistant waterhemp control has been achieved in corn, soybean, and wheat when integrated weed management practices were implemented. Further results will be Weed resistance management WATERHEMP, CANADA FLEABANE, AND EMERGING ISSUES Owen Roberts WATERHEMP IN SOYBEAN FIELD. released in 2021, when seed density is determined following the first three-year cycle of the corn-soybean-wheat rotation in this study. Meanwhile, to help control multiple-herbicide- resistant waterhemp in a corn-and-soybean rotation, Sikkema recommends that farmers consider a two-pass weed-control program. He says to start with an effective soil-applied herbicide, such as Acuron or Integrity for corn, and Fierce, TriActor, Authority Supreme, Boundary or Bifecta for soybean. Then, follow with a post-emergence herbicide if needed, such as Acuron, Shieldex + atrazine, Converge + atrazine, Callisto + atrazine, Enlist or dicamba for corn; Reflex, Blazer or Hurricane for IP or RR soybean, Enlist Duo for Enlist soybean, and dicamba or Tavium for Xtend soybeans. Overall, he says an integrated approach that includes herbicides — but not exclusively — is best. “As Dr. Andrew Kniss of University of Wyoming states, at some point, we need to stop looking to herbicides as the solution to a problem created by herbicides,” he says. HERBICIDE STRATEGIES Glyphosate-resistant Canada fleabane can be another costly problem to have on your farm. But research conducted by Sikkema shows that greater than 90 per cent control is possible. He launched an on-going research project a decade ago when the first glyphosate-resistant weed biotype was identified in Ontario. “The results from this research will provide Ontario grain farmers with new knowledge to improve the management of glyphosate- resistant Canada fleabane on their farms.” For glyphosate-resistant Canada fleabane control in corn with herbicides applied preplant, Research